Conventional rectifiers have relatively low power factors which limit the power that can be drawn from an ac line to a fraction of the rating of the line. Furthermore, highly distorted ac line currents are drawn by these conventional rectifiers, often causing interference with other electrical equipment in addition to equipment overcurrents and overvoltages. Techniques for improving power factor include passive waveform shaping methods, i.e. using input filters, and active methods, i.e. using boost or buck converter topologies. Such conventional active methods of obtaining high power factor generally employ a completely separate up-front converter to attain the high power factor followed by a dc-to-dc converter to produce the desired regulated dc output voltage. Thus, the power is converted twice, which is costly and inefficient. Moreover, the upfront converter must convert the entire delivered power. In fact, it must convert a peak power equal to twice the average power delivered.
A power conversion system employing a single power stage while operating at high power factor is described in commonly assigned U.S. Pat. No. 4,642,745 issued on Feb. 10, 1987 to R. L. Steigerwald and W. P. Kornrumpf, which patent is incorporated by reference herein. The power conversion system of the Steigerwald and Kornrumpf patent includes: a full-wave ac rectifier; a dc-to-ac converter; a transformer having a primary winding, a closely-coupled secondary output winding and a loosely-coupled secondary boost winding; and a resonant capacitor coupled to the secondary boost winding. The secondary output winding is controlled by pulse width modulation (PWM), and the secondary boost winding is controlled by frequency modulation, thus allowing relatively independent control of the input current and output voltage. However, since the secondary boost winding and the primary boost winding of the Steigerwald and Kornrumpf patent are coupled, albeit loosely, any adjustment in the dc output voltage by the PWM control affects the input current waveform. Therefore, although the power supply of the hereinabove cited Steigerwald and Kornrumpf patent results in relatively high power factor using only one power stage, it may be desirable in some applications to further increase power factor and otherwise improve performance (e.g., by lowering the output ripple current) by completely decoupling the boosting converter and the power supply output voltage.